Gridsplitter not resizing

I want the GridSplitter to make the TreeView larger to make it easier to see the content of the TreeView. Can not seem to find the right place to put the Splitter

<Grid.ColumnDefinitions>
    <ColumnDefinition/>
    <ColumnDefinition Width="auto"/>
    <ColumnDefinition Width="auto"/>
</Grid.ColumnDefinitions>

<Grid.RowDefinitions>
    <RowDefinition Height="auto"/>
    <RowDefinition/>
    <RowDefinition Height="auto"/>
    <RowDefinition />
    <RowDefinition Height="auto"/>
    <RowDefinition Height="auto"/>
</Grid.RowDefinitions>

<TextBlock Grid.Column="0" Grid.Row="0" Text="JSON String" Margin="5"/>
<TextBox Grid.Column="0" Grid.Row="1" Grid.ColumnSpan="3"/>

<GridSplitter Grid.Column="0" Grid.ColumnSpan="3" Grid.Row="2" Height="5"         ResizeDirection="Rows" ResizeBehavior="PreviousAndNext" HorizontalAlignment="Stretch"/>

<TextBlock Grid.Column="0" Grid.Row="3" Text="C# Type" Margin="5"/>
<TreeView Grid.Column="0" Grid.Row="4" Grid.ColumnSpan="3" x:Name="outputTree" Margin="5"/>